The present invention relates to a process for preparing crosslinked ion exchangers with a homogeneous network structure based on unsaturated aliphatic nitrites in the presence of film-forming protective colloids, where the process carried out is not a seed/feed process.
Copolymerization of two or more monomers generally gives rise to polymers whose composition changes to some extent as conversion increases. Depending on the type of polymerization, two types of heterogeneity can be distinguished.
If the lifetime of the active group on the polymer chain is the same as the duration of the polymerization, as in the case of anionic polymerization, the composition changes along the chain. At any given juncture during the polymerization all of the polymer chains have the same overall composition.
If the lifetime of the active group on the polymer chain is significantly shorter than the overall reaction time, the composition of the polymer chains changes as the conversion proceeds in the polymerization. This second type of heterogeneity is typical of free-radical polymerization reactions in which the lifetime of the growing polymer radicals is in the region of seconds and the polymerization time in the region of hours.
This heterogeneity is associated with disadvantages for many applications. In the case of adsorber resins and ion exchangers, which are crosslinked bead polymers, non-crosslinked or very weakly crosslinked fractions are highly undesirable. These always arise if there is more than proportional incorporation of crosslinking agent, the concentration of which decreases markedly as the polymerization proceeds. A typical example of this is the acrylonitrile/divinylbenzene combination. Bead polymers and/or weakly acidic ion exchangers prepared from this combination contain considerable amounts of non-crosslinked polymers, which can exude and give products which are mechanically and osmotically unstable. Attempts are made to compensate for the drop in concentration of the crosslinking agent during the polymerization by either using a second crosslinking agent which is incorporated only slowly or feeding further amounts of the crosslinking agent whose concentration drops rapidly (in this case divinylbenzene). The second crosslinking agents used in industry are di- or triallyl compounds, such as 1,7-octadiene or trivinylcyclohexane. These substances react only incompletely and have to be carefully removed so that the resin does not cause difficulties in downstream applications. Feeding further amounts of the crosslinking agent is difficult and requires a sophisticated feed strategy, and there is also a limitation on the selection of the suspension stabilizers. Protective colloids frequently used in bead polymerization, such as gelatins, polyvinyl alcohol, or cellulose derivatives, are film-forming substances which are unsuitable since they inhibit the diffusion of the crosslinking agent into the polymerizing beads (EP-A 98,130).
There is therefore great interest in any process which does not have the disadvantages described above. Surprisingly, it has been found that unsaturated aliphatic nitrites, such as acrylonitrile, in combination with di- and polyvinyl ethers of alkanediols or of glycols build up a very homogeneous network and give products which do not have the disadvantages described above. It is possible to dispense with feeding of further amounts of the crosslinking agent and with the use of a second crosslinking agent.
The use of di- or polyvinyl ethers for crosslinking polymers is prior art.
EP-A 10,265 describes the preparation of synthetic resins based on crosslinked copolymers of mono- and polyvinyl compounds. A substantive feature of the invention is the joint use of two crosslinking agents, as stated in the Abstract: methacrylates of polyhydric alcohols and/or aromatic polyvinyl compounds and unsaturated hydrocarbons which have at least two allyl groups in the molecule and/or polyvinyl ethers of polyhydric alcohols. There is also an indication of the use of acrylonitrile in the bead polymers, but susceptibility to hydrolysis is described in the text as a serious disadvantage of known synthetic resins based on acrylonitrile.
U.S. Pat. No. 3,586,646 describes the use of divinyl ethers for preparing spongy cation exchangers having groups selected from the class consisting of sulfonic acid groups, carboxylic acid groups, phosphoric acid groups, and phosphonic acid groups, in the presence of a porogen (an organic solvent which dissolves the monomer but is a precipitant for the polymer). However, U.S. Pat. No. 3,586,646 does not describe the use of unsaturated nitrites in the bead polymers.
EP-A 98,130 describes the preparation of crosslinked copolymer beads by a seed/feed process, and describes the copolymer beads themselves and their use as adsorbers or, after introduction or addition of functional groups, as ion-exchange resins. Monomers mentioned for the seed and/or the feed include nitriles of acrylic and/or methacrylic acid, and crosslinking agents mentioned include polyvinyl ethers of glycol, glycerol, pentaerythritol, resorcinol or monothio- or dithio derivatives of glycols. The preparation of the seed/feed particles requires the absence of protective colloids, or a drastic reduction in the amount of these, as they prevent, or dramatically delay, the absorption of the feed component by the seed particles. The particles prepared according to EP-A 98,130 also show multistage swelling behaviour in toluene and birefringence in the form of a maltese cross under polarized light.
None of the patent applications/patents cited gives any indication as to how monomers and crosslinking agents have to be selected in order to obtain crosslinked bead polymers with a homogeneous network structure, particularly in the presence of film-forming protective colloids.
The object of the present invention was to prepare ion exchangers, preferably weakly acidic cation exchangers, having a homogeneous network structure and based on unsaturated aliphatic nitrites in the presence of film-forming protective colloids.
The present invention achieves this object in providing a process for preparing crosslinked ion exchangers comprising
(a) polymerizing unsaturated aliphatic nitrites with di- or polyvinyl ethers as crosslinking agents and also with initiators in suspension in the presence of protective colloids, but not by a seed/feed process, to give bead polymers, and
(b) functionalizing the resultant bead polymers to give ion exchangers.
If desired, other monovinyl compounds and/or other crosslinking agents and/or porogen may be added to the polymerization reaction mixture.
The bead polymers prepared by step (a) are likewise provided by the present invention. They have a homogeneous network structure and can be used as adsorber resins.
In step (b) the bead polymers are functionalized by customary methods known to those skilled in the art to give ion exchangers, particularly weakly acidic cation exchangers. To prepare weakly acidic cation exchangers, the bead polymers obtained in step (a) are hydrolysed under alkaline conditions, subjected to ion-exchange if desired, and purified. Alkaline hydrolysis has proven to be a particularly effective and economic practical process. The weakly acidic cation exchangers preferably obtained by the novel process exhibit a particularly high capacity.
For the purposes of step (a) of the present invention, unsaturated aliphatic nitrites are defined by the general formula (I) 
wherein each of A, B, and C, independently of the others, represents hydrogen, alkyl, or halogen.
For the purposes of the present invention, the term xe2x80x9calkylxe2x80x9d refers to straight-chain or branched alkyl having from 1 to 8 carbon atoms (preferably from 1 to 4 carbon atoms). For the purposes of the present invention, the term xe2x80x9chalogenxe2x80x9d refers to chlorine, fluorine, or bromine.
For the purposes of the present invention, preferred nitriles are acrylonitrile and methacrylonitrile, and the use of acrylonitrile is particularly preferred.
For the purposes of step (a) of the present invention, suitable divinyl ethers are compounds of the general formula (II) 
wherein
R represents a radical selected from the group consisting of CnH2n, (CmH2mxe2x80x94O)pxe2x80x94CmH2m, and CH2xe2x80x94C6H4xe2x80x94CH2,
n is xe2x89xa72,
m is from 2to 8,and
p is xe2x89xa72.
For the purposes of step (a) of the present invention, suitable polyvinyl ethers are trivinyl ethers of glycerol or trimethylolpropane or tetravinyl ethers of pentaerythritol.
It is preferable to use divinyl ethers of ethylene glycol, di-, tetra-, or polyethylene glycol or butanediol or polyTHF, or the tri- or tetravinyl ethers. Particular preference is given to the divinyl ethers of butanediol and diethylene glycol.
If desired, use may also be made of other monovinyl compounds and/or other crosslinking agents.
For the purposes of the present invention, suitable monomers are styrene and styrene derivatives, acrylic acid, and methacrylic acid and their esters, amides, and anhydrides, vinyl chloride, vinylidene chloride, vinyl acetate, and vinylpyrrolidone.
For the purposes of the present invention, suitable other crosslinking agents are divinylbenzene, di- and poly(meth)acrylates of glycols, of alkanediols having three or more carbon atoms, glycerol, trimethylolpropane, or pentaerythritol, 1,7-octadiene, and trivinylcyclohexane. Divinylbenzene is preferred. To increase the porosity of the beads use may be made of porogens. Suitable porogens in the novel process are organic solvents in which the monomers are soluble but which are poor solvents and, respectively, swelling agents with respect to the polymer produced. Examples of these compounds are those selected from the group of ketones, such as methyl isobutyl ketone or methyl ethyl ketone, from the group consisting of hydrocarbons, such as hexane, octane, isooctane, and isododecane, or from the group consisting of alcohols having four or more carbon atoms, such as butanols (Farbenfabriken Bayer, DBP 1,045,102 [1957], and DBP 1,113,570 [1957]).
The preparation of the weakly acidic cation exchangers preferred according to the present invention can be carried out by
(xcex1) polymerizing unsaturated aliphatic nitrites of the general formula (I) with di- or polyvinyl ethers and, optionally, further crosslinking agents and initiators in suspension in the presence of protective colloids, to give bead polymers,
(xcex2) subjecting the bead polymer to alkaline hydrolysis (preferably in an autoclave), and
(xcex3) subjecting the hydrolyzed bead polymers to ion-exchange from the salt form (preferably Na form) into the H form by dilute mineral acid (preferably 10% strength H2SO4), preferably in a column,
(xcex4) optionally, purifying the H form of the bead polymers with water in an autoclave and, finally,
(xcex5) optionally, classifying the bead polymers into desired particle size ranges in a column.
The bead polymer may also be screened, if necessary.
The suspension polymerization according to the invention is carried out in the presence of protective colloids and, if desired, in the presence of dispersing agents. It is advisable to use compounds that are stable in the presence of salts, such as hydroxyethylcellulose or condensation products of naphthalenesulfonic acid with formaldehyde in the presence of the alkali metal or alkaline earth metal salts of hydrohalic acids or of sulfuric acid (particularly preferably of NaCl, Na2SO4, or CaCl2). The salts reduce the solubility of the monomers in the aqueous phase. To reduce agglomeration, pH buffers made from borate or phosphate may be added. For the purposes of the present invention, suitable free-radical initiators are peroxides, hydroperoxides, peresters, azo initiators and other initiators with half-life times txc2xd of 1 hour at from 60 to 140xc2x0 C. (AKZO Nobel company publication: Initiators for High Polymers). Examples of preferred initiators suitable for the novel process are peroxy compounds, such as dibenzoyl peroxide, dilauroyl peroxide, bis(p-chlorobenzoyl) peroxide, dicyclohexyl peroxydicarobnate, tert-butyl peroctoate, tert-butyl peroxy-2-ethylhexanoate, 2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane, or tert-amyl peroxy-2-ethylhexane, or else azo compounds, such as 2,2xe2x80x2-azobis(isobutyronitrile) or 2,2xe2x80x2-azobis(2-methylisobutyronitrile). Preferred initiators have half-life times txc2xd of 1 hour at from 75 to 110xc2x0 C. It is particularly preferable to use dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate, or azoisobutyronitrile.
The polymerization is preferably carried out in two stages. The main reaction is first completed at a temperature of from 50 to 80xc2x0 C. and then completed at an elevated temperature. It is particularly preferable to carry out a polymerization at from 65 to 75xc2x0 C., followed by an increase of the temperature to 85-100xc2x0 C.
To prepare the weakly acidic ion exchangers preferred according to the present invention, the bead polymers are hydrolyzed by aqueous sodium hydroxide in an autoclave. Preference is given to alkaline hydrolysis with aqueous/methanolic sodium hydroxide. Another possibility is hydrolysis at atmospheric pressure by aqueous/methanolic sodium hydroxide. However, acid hydrolysis by mineral acids is also possible, based on the prior art. Reference is made to EP-A 406,648 concerning the conduct of the hydrolysis at superatmospheric pressure. Preference is given to procedure 1, in which the bead polymer forms an initial charge and the aqueous sodium hydroxide is fed. Once the hydrolysis is complete, the resin is washed through, subjected to ion-exchange using 10% strength H2SO4 at 90xc2x0 C. and then washed until neutral. For purification, the resin is treated with water or, respectively, steam at an elevated temperature. Fine-particle constituents may then be removed in a classifying column.
The novel bead polymers from step (a) and (xcex1), respectively, have wide application as adsorbers.
The novel weakly acidic macroporous cation exchangers may be used in the food or drink industry or for drinking water treatment, for example. They are particularly suitable for removing cations/hardness from drinking water (e.g., in household filters) and for decarbonizing drinking water, or else decarbonizing liquids used as food or drink or in preparing food or drink. Other important applications are removal of cations/hardness from sugar solutions or solutions of organic products, e.g., of beet sugar, of cane sugar, or of starch sugar or, respectively, glycerol, gelatin, etc., desalination of water during preparation of ultrahigh-purity water, decarbonization of service water (in the cocurrent process), in association with a strongly acid cation exchanger for the desalination of water for industrial steam generation, as a buffer filter downstream of desalination plants for binding alkali metal ions, in the neutralization of regeneration wastewater from desalination plants, in the sodium form for binding heavy metals, such as copper, nickel or zinc, from solutions at pH greater than 5 in the absence of calcium ions, and of complexing agents.
The novel weakly acidic macroporous cation exchangers may also be used to remove polar or nonpolar compounds or heavy metals from aqueous or organic solutions from the chemical industry, electronics industry, food or drink industry, waste-disposal industry, or waste-reclamation industry.
The novel bead polymers and/or ion exchangers may in particular be used
for removing polar compounds from aqueous or organic solutions,
for removing polar compounds from process streams from the chemical industry,
for removing organic components from aqueous solutions or from gases, for example, from acetone or chlorobenzene, or
for removing heavy metals or precious metals, arsenic, or selenium, from aqueous solutions.
For the purposes of the present invention, heavy metals or precious metals are the elements of the Periodic Table with serial numbers from 22 to 30, from 42 to 50, and also from 74 to 83.
The novel ion exchangers and bead polymers, moreover, may be used for purifying or treating water from the chemical industry or electronics industry, or else from the food or drink industry, particularly for preparing ultrahigh-purity water, ultrahigh-purity chemicals or preparing starch or hydrolysis products thereof.
The novel ion exchangers and bead polymers may moreover be used for purifying wastewater streams from the chemical industry, or else from waste-incineration plants. Another application of the novel adsorbers is the purification of landfill run-off water.
The novel ion exchangers and bead polymers may also be used for treating drinking water or groundwater. However, the novel bead polymers and/or ion exchangers may also serve as a starting material for other poly(meth)acrylic acid derivatives, by reactions such as:
reaction with 1-N,N-dimethylamino-3-aminopropane (amine Z) to give acrylamide resins, which products may be partially or completely quaternized and serve for treatment of water or of sugar,
reaction with polyamines, such as diethylenetriamine or triethylenetetramine, to give highly sulfate-selective resins, or
reaction with amino sugars (e.g. N-methylglucamine) to give boron-selective resins.
The novel ion exchangers are prepared in a number of steps and are illustrated using the examples below.